Sintering aids for producing alumina dielectric compositions cofireable with palladium

ABSTRACT

Sintering aids for producing dielectric compositions of alumina which may be sintered at or below 1,500*C. and, hence, may be cofired with palladium metallizations. These sintering aids are glass frits made from certain critical combinations of oxides. Also dielectric compositions of 75-97 percent alumina and 3-25 percent sintering aid, and articles comprising ceramic dielectric substrates made from those dielectric compositions.

United States Patent 11 1 [451 Apr. 2, 1974 Conwicke [54] SlNTERlNG AIDSFOR PRODUCING 2,956,219 10/1960 Cianch 317/258 L MIN DIE C COMPOSITIONS3,019,112 1/1962 Doucette 106/734 ,g 7,35 10/1966 lkeda etal 106/53COFIREABLE WITH PALLADIUM 3,558,332 1/1971 Bachaman 106/73.4 [75]Inventor: Joel Alfred Conwicke, Wilmington, 3,586,522 6/1971 Hoffman106/39.6 De] 3,673,092 6/1972 Dietz 106/733 [73] Assignee: E. I. du Pontde Nemours and Company, Wilmington, Del. Primary ExaminerA. B. CurtisFiled Oct 16 1972 Assistant ExaminerMark Bell [21] Appl. No.: 297,733

Related US. Application Data [57] ABSTRACT [62] Division of 101,166 1970Sintering aids for producing dielectric compositions of abandonedalumina which may be sintered at or below 1,500C.

and, hence, may be cofired with palladium metalliza- [2%] :LS. Cll.l3036/273A, 252/??8 tions. These simering aids are glass frits made fromg g 2 certain critical combinations of oxides. Also dielectric 1 le 0can l7 4 compositions of 75-97 percent alumina and 325 per- 317/258252/508 cent sintering aid, and articles comprising ceramic dilt' btt dfthsedielet'como- References Cited siggnrc su s ra es ma e mm o 0 me pUNITED STATES PATENTS 5/1959 Bickford 106/46 11 Claims, No DrawingsSINTERING AIDS FOR PRODUCING ALUMINA DIELECTRIC COMPOSITIONS COFIREABLEWITH PALLADIUM This is a division of application Ser. No. 101,166, filedDec. 23, 1970.

BACKGROUND OF THE INVENTION This invention relates to ceramic substratesfor electronic circuitry and, more particularly, to alumina substrateswith low dielectric constants.

Ceramic dielectric substrates having a high alumina content are usedextensively in the electronics industry as the support for hybridmicroelectronic circuitry. Such high alumina ceramics, commonly above 90percent alumina, are chosen due to mechanical strength and thermalconductivity, as well as their dielectric character. Such substrates arecommonly made by a tape process, whereby a flexible sheet is made bydispersing alumina in a suitable binder/solvent system, depositing thesame on a support, doctor blading the slurry, drying it, and thenstripping the alumina tape from the support. The tape is then cut orpunched to size. Depending on composition, the cut pieces are fired inan inert, reducing or oxiding atmosphere at elevated temperature,generally 1,650C., to densify the ceramic to near-theoretical density;porosity must be kept below 8 percent to ensure acceptable properties.

Multilayer ceramic substrates with buried conductor metallizations areemployed to achieve maximum integrated circuit density in hermeticelectronic packages. High alumina compositions which must be air firedat 1,650C. require the use of platinum as buried conductors, since lessnoble or refractory metals either melt or oxidize, resulting indestruction of electrical continuity. Considerable economic advantagescan be gained by using palladium metallizations. However, the meltingpoint of palladium (1,550C.) requires a dielectric tape compositionwhich densifies below 1,550C., preferably at or below 1,500C.Furthermore, the availability of ceramic densifying at or below 1,500C.would simplify processing and reduce costs.

Exemplary of art regarding sintering of alumina is the literature citedin Chapter 16, entitled Sintering, appearing in the American CeramicSocietys Alumina as a Ceramic Material, pages 127-134, 1970. Talc (amagnesium silicate) and magnesium oxide are common additives.

There is a need for sintering aids which will make high alumina ceramicscompatible with (cofireable with) palladium metallizations to producesubstrates exhibiting high thermal conductivity, mechanical strength andsurface smoothness.

SUMMARY OF THE INVENTION This invention provides an alumina sintering ordensification aid useful in dielectric compositions which may besintered at or below 1,500C. and, hence, may be cofired with palladiummetallizations. The sintering aid is a glass frit of the composition setforth in Table l, which sets forth both operable and preferredcomposition ranges.

TABLE 1 GLASS SINTERING AID FOR ALUMINA Weight Percentage ComponentOperable Range Preferred Range SiO, -50 -50 A1,O 5-20 5-12 Alkaline14-26 14-26 Earth Oxides TiO, 1-15 5-12 ZnO, l-lO 2-8 PbO 1-10 2-8sheets with binders) in air at temperatures of 1,500C.

or below, usually l,450l ,500C. The fired substrates, employed with bothburied and top surface palladium metallizations, are dense ceramicswhich exhibit very smooth surface finish, high mechanical strength, andlow dielectric loss.

DETAILED DESCRIPTION The sintering (or densification) aids of thepresent invention are glass frits which are added to alumina to helpdensify alumina at temperatures below the melting point of palladiummetallizations, thus permitting cofiring of the metallized substrates attemperatures below the melting point of palladium, and usually in therange 1,4501,500C. The sintering aid causes the substrate to coalesceinto a nonporous structure and, thus, produce a dense substrate.

The glass frit is of the composition set forth in Table l and isprepared from the prescribed amounts of the respective oxides orprecursors of the oxides [e.g., A1 0 may be supplied to the batch asAl(OH) by conventional techniques of melting the oxides and/or oxideprecursors to a clear mass and then pouring the molten mass into coldwater, following by ball milling of the frit to the desired particlesize.

By the term alkaline earth oxides, one or more of which may be presentin the sintering aids of the present invention so long as the totalweight percent thereof in the sintering aid is in the range 14-26percent, is meant the oxides of Ca, Mg, Sr and Ba.

The sintering aid of the present invention is normally ground withalumina to prepare a dielectric composition with particles having anaverage particle diameter in. the range 1-5 microns, preferably 1-3microns, in any case with substantially no particles larger than 10microns in diameter.

The dielectric compositions of the present invention consist essentiallyof -97 percent of alumina and 11-25 percent of the above-described glasssintering aid (by weight). Especially useful high alumina dielectriccompositions consist essentially of 91-97 percent alumina and 3-9percent sintering aid. The alumina, prior to use, must be in finelydivided form, that is, the average particle diameter of the alumina is1-5 microns, preferably l-3 microns, substantially no particles having adiameter greater than microns. Reynolds RC-l52 DBM alumina is apreferred alumina, having an average particle diameter of about 1.6microns. Another operable alumina is Reynolds RC122, having an averageparticle diameter of about 5 microns. It is, therefore, clear thatsubmicron sized reactive alumina powders, difficult to use in bindersystems, are not employed in the dielectric compositions of the presentinvention.

Normally, the desired amounts of alumina and a given sintering aid areball milled together with suitable wetting agents and dispersing aids,such as trisodium polyphosphate, methyl cellulose or one of the Rohm andHaas Tamol dispersing agent series.

Ceramic articles may be made from the dielectric compositions of thepresent invention by various methods, including that of Park U.S. Pat.No. 2,966,719. Preferably, the sintering aid and alumina in finelydivided form are thoroughly mixed with a binder system to form a slurryand the slurry mixture is deposited on a smooth carrier surface (forexample, polyethylenecoated Du Pont Mylar" polyester film) as a sheet ortape by any conventional coating and/or extrusion technique such asdoctor blading, casting, etc. Then the article is dried by anyconventional drying procedure, e.g., infrared, air-dry, oven heat.Drying may be accelerated by heating to a temperature in the range 50l50C.

The binder system employed may be based on a volatile organic liquid orwater, preferably upon water. A preferred binder system is thewater-based resin system claimed in copending application S.N. 850,617,filed Aug. 15, 1969, now abandoned. The resultant dry, flexible tape orsheet is stripped from the carrier surface and punched or cut to thedesired sheet dimensions.

Usually the unsintered cut sheets will be used to make multilayermonolithic internally metallized structures. Thus, the sheets arefurther processed to provide via holes which, when filled withmetallization and fired, will provide electrical continuity between thevarious layers. The sheets are then metallized as required, and the viaholes are filled with metallization. The sheets are then laminated underheat and/or pressure, and the laminate is then fired to burn out binder(e.g., it is heated from room temperature to 600C. in 4 hours), and thensintered in air at 1,450l,500C. The duration of the heating process issufficient to form nonporous articles, and is normally in the range 1 to4 hours. Often about 2 hours is required to densify the article togreater than 95 percent of the theoretical density and and, thus,achieve the desired electrical, mechanical and thermal properties. Asdesired, exposed (top surface) metallizations can be post-fired orcofired with the laminate.

The mechanism by which the glass sintering aids of the present inventiondensify alumina during firing has not been conclusively studied.However, it is thought that sintering takes place by a liquid-phasemechanism. The glass compositions of this invention are molten at1,500C. and remain liquid as dissolution of non-fritted aluminaparticles occurs during sintering. Thus, the alumina content of theglass phase rises during sintering and causes formation, upon cooling,of a coalesced densified structure of alumina particles in a glassymatrix. Assuming that this mechanism is correct, it is seen that it isimportant that the glass densification aid melt at or below 1,500C.

Examples and Comparative Showings This invention is illustrated by thefollowing examples (designated by Roman numerals), and is contrastedwith inoperative compositions (designated by capital letters). In thefollowing experiments and elsewhere in this specification and theappended claims, all parts, ratios and percentages of materials orcomponents are by weight.

Fired substrates were examined for open porosity visually by a dyeabsorption technique using Magnaflux Corp. Zyglo dye as the penetrant.The fired substrate was immersed in the penetrant for 15 minutes, thenrinsed in cold running water for 10 minutes to remove excess dye. Thesubstrates were then inspected under U.V. light for indications of dyepenetration.

Apparent density, surface smoothness, flexural strength, electricaldissipation factor and volume resistivity of buried palladiummetallizations were each determined, but only on those fired examplespassing the above-described qualitative test for open porosity.

Apparent density was determined according to the Archimedes method usingwater or methanol. Surface smoothness was studied using a Model 150Selvite Bush Surface Analyzer to determine surface profile traces.Flexural strength was determined according to ASTM D790. Dissipationfactors were measured at l KHL; volume resistivities of the buriedpalladium metallizations were determined from the resistance andcrosssectional area of a known length of buried metallization.

The glass frits of Table II were prepared as follows. The oxides wereweighed out and mixed, except that A1 0 was supplied to the batch asAl(OH The mixture was melted at the temperature indicated in Table ll ina kyanite crucible to form a clear homogeneous melt; the molten glasswas poured into cold, running water to form a frit; then the frit wasground in a ball mill jar equipped with the normal complement (halffull) of grinding medium (ceramic balls) and the proper weight of wateruntil less than 1 percent residue was retained on a 400-mesh sieve (U.S.standard scale).

The respective amounts of the glass frit and Al O (Reynolds RC-l22alumina or RC-l52 alumina, as prescribed in Table III) indicated inTable III for the respective examples and comparative showings, totaling300 g., were placed in a one quart borundum percent Al O 12 percentSiO,, 2 percent MgO and l percent CaO) ball mill jar half filled with3/l6-inch borundum grinding media. To the jar was added l20 g. of a 1percent aqueous solution of Methocel (methyl cellulose grade 90, DowChemical Company) and 0.5 g. of trisodium polyphosphate wetting agent.The mixture was milled for 16 hours and the resultant slurry wasdeaerated under vacuum using 1 or 2 drops of octyl alcohol to break upany foam.

Binder was added to the slurry as follows: g. of slurry was stirredduring the slow addition thereto of 29 g. of Rohm and Haas I-lA-l2binder (an acrylic ester) and 5 drops of 25 percent aqueous solution ofRohm and Haas Tamol SN.

The resultant slurry was then cast on polyethylenecoated Mylar"polyester sheets as 22-mil thick tapes and air dried. From the resultantdry, flexible unfired tape were punched 1.2-inch squares. The squareswere metallized with palladium by a thick film technique by printing a124 square pattern through a 200-mesh screen (U.S. sieve scale), thenlaminated in a steel die at 5,000-25,000 p.s.i. The laminated body wasthen fired in air at 600C. for 4 hours to burn off organic materials,then densitied at a firing temperature in the range l,450-l,500C. (asindicated in Table III) for 2 hours.

Open porosity was not observed with the fired examples of invention(Runs I-IV), but was observed with comparative showings A-E. Furthercharacterization of the examples of the invention is recorded in TableIII.

I claim: I. A dielectric composition for preparing low dielec- TABLE upercent alumina and 3-25 percent of a glass frit as an alumina sinteringaid, said glass frit consisting essentially of,

30-50 percent SiO 5-20 percent A1 14-26 percent One or more alkalineearth oxides 1-15 percent TiO 1-10 percent ZnO 0 1-10 percent PbO 2. Adielectric composition according to claim 1 wherein the glass fritconsists essentially of, by weight,

40-50 percent- SiO -12 percent A1 0 14-26 percent One or more alkalineearth oxides 5-12 percent TiO 2-8 percent ZnO 2-8 percent PbO BATCHCOMPONENTS OF GLASS SINTERING AIDS (WT.

MELTING TEMPERATURE Component Glass No.

SiO, 45 50 50 45 45 45 54 M g0 10 30 12 20 A 8 C210 10 10 I5 20 8 AI QI0 10 5 10 5 20 TiO, l0 l0 10 10 10 10 ZnO 5 5 5 8 PbO 5 5 10 Melting1500 1550 1500 1450 1500 1500 1550 Temp. 7

TABLE III COMPOSITIONS AND PROPERTIES OF ALUMINA SUBSTRATES Examples ofInvention Comparative Showings I II III IV A B C D E Unfritted A1 0 273288 280 255 270 270 270 270 282 R C Type 152 152 152 152 122 122 122 122152 Glass Wt. (g.) 27 12 20 30 30 30 30 18 Wt. Glass in 9 4 6.3 85 10 1010 10 6 Total Glass Plus Unfritted A1 0; Total A1 0 in Al O l 91.8 96.494.0 86.5 90.5 92.4 91.0 90.5 95.2 Glass Mixture (Wt. Firing Temp. (C.)I440 I500 I500 I500 1500 1500 I500 I500 1500 Open Porosity No No No NoYes Yes Yes Yes Yes Apparent Density 3.66 3.71 3.69 (g-IcC.) FlcxuralStrength 60,000 50,000 50,000 (p.s.i.) Surface smoothness 6 20 21 15(microinches, center line average) Qissipaliqn Factor .0005 .0005 .0006(absoIute) Volume Resistivity of 15 15 15 Buried Pd (IL-Ohm. cm.)

tric eanstsfiiammifisamime substrateswliieh' compo sition is cofireablewith palladium metallizations, said composition consisting essentiallyof, by weight, -97

3. A dieleetric composition according to claim 1 consisting essentiallyof, by weight, 91-97 percent alumina and 3-9 percent of said glass frit.

4. A dielectric composition according to claim 3 wherein the glass fritconsists essentially of, by weight,

7. A dielectric composition according to claim I having particles withan average diameter in the range 1-5 microns, with substantially noparticles being larger than 10 microns in diameter.

8. A dielectric composition according to claim 7 wherein the averageparticle diameter is in the range 1-3 microns.

9. A ceramic article comprising a substrate consisting essentially ofthe sintered composition of claim l.

10. A ceramic article comprising a substrate consisting essentially ofthe sintered composition of claim 2.

1 l. A ceramic article comprising a substrate consisting essentially ofthe sintered composition of claim 3.

2. A dielectric composition according to claim 1 wherein the glass fritconsists essentially of, by weight, 40-50 percent SiO2 5-12 percentAl2O3 14-26 percent One or more alkaline earth oxides 5-12 percent TiO22-8 percent ZnO 2-8 percent PbO
 3. A dielectric composition according toclaim 1 consisting essentially of, by weight, 91-97 percent alumina and3-9 percent of said glass frit.
 4. A dielectric composition according toclaim 3 wherein the glass frit consists essentially of, by weight, 40-50percent SiO2 5-12 percent Al2O3 14-26 percent One or more alkaline earthoxides 5-12 percent TiO2 2-8 percent ZnO 2-8 percent PbO
 5. A dielectriccomposition according to claim 4 having particles with an averagediameter in the range 1-5 microns, with substantially no particles beinglarger than 10 microns in diameter.
 6. A dielectric compositionaccording to claim 5 wherein the average particle diameter is in therange 1-3 microns.
 7. A dielectric composition according to claim 1having particles with an average diameter in the range 1-5 microns, withsubstantially no particles being larger than 10 microns in diameter. 8.A dielectric composition according to claim 7 wherein the averageparticle diameTer is in the range 1-3 microns.
 9. A ceramic articlecomprising a substrate consisting essentially of the sinteredcomposition of claim
 1. 10. A ceramic article comprising a substrateconsisting essentially of the sintered composition of claim
 2. 11. Aceramic article comprising a substrate consisting essentially of thesintered composition of claim 3.